I. Field of the Invention
This invention relates to a process of conversion of hydrocarbons via gasification into two high pressure gas streams: a hydrogen-rich stream and a carbon-monoxide-rich stream. More specifically, this invention relates to the use of a two-zone predominantly molten iron or molten iron alloy system in conjunction with the above gasification conversion.
II. Discussion of the Prior Art
Two-zone molten iron gasifiers are disclosed by:
U.S. Pat. No. 1,803,221 (1931) to Tyrer describes hydrogen-rich gas production by feeding methane or a methane-steam mixture into one molten iron zone below the surface of the metal, thereby assuring complete reaction of the gaseous feed. The carbon which dissolves in the molten iron in the first zone is burned out of the molten iron in the second zone with an oxygen containing gas. Additional oxygen containing gas may be added to the combustion products leaving the second zone to completely oxidize to carbon dioxide any carbon monoxide remaining.
The disadvantages of the process described in this patent include:
The feedstocks are limited to hydrocarbon gases such as methane and do not include lower value hydrocarbon liquids or solids.
Operating pressure is nominally atmospheric pressure which is less economical to operate than equipment producing hydrogen at elevated pressures; two atmospheres and above.
The importance of controlling the carbon level in the molten iron is not considered and thus production of only lower purity hydrogen-rich gas is possible. If a minimum carbon level of at least 0.3% is not maintained, excess iron oxide will form in the molten iron during oxidation and will be converted to carbon monoxide and dilute the hydrogen-rich gas when hydrocarbon feeds are introduced to the hydrogen-rich gas producing first zone.
U.S. Pat. Nos. 4,187,672 (1980) and 4,244,180 (1981) to Rasor describe a hydrocarbon gasification process in which solid hydrocarbons such as coal are introduced on the surface of one molten iron bath zone in which high temperature cracking of the hydrocarbons into lighter molecular weight materials takes place with residual carbon being dissolved in the molten iron. The cracked hydrocarbon products are removed via outlets in the shaft through which the feed hydrocarbon solids enter the molten iron. The molten iron containing the carbon is transferred to the second molten iron zone in which an oxygen containing gas is introduced to convert the carbon into carbon monoxide and raise the temperature of the iron for transfer back to the carbonization section. The carbon monoxide is further oxidized above the molten iron bath and the heat recovered via a boiler or similar system. Sulfur, if present in the feed, is removed via slag formation on top of the molten iron. The disadvantages of the process described in this patent include.
The feedstocks are limited to solid hydrocarbons such as coal and do not include lower value hydrocarbon liquids or gases.
Since the solid hydrocarbon feeds are introduced above the surface of the molten iron, cracking of the feeds occurs such that a very impure hydrogen gas stream is produced because of the presence of cracked hydrocarbon gases.
Since the product gas from the oxidation zone is further oxidized for energy recovery in, for example, a steam boiler, no attempt is made to produce a carbon monoxide-rich gas.
Sulfur removal from the solid feed via reaction with and removal of slag from the equipment is complicated and expensive.
Operating pressure is nominally atmospheric pressure, which is less economical to operate that equipment producing hydrogen at elevated pressures, two atmospheres and above.
The importance of controlling tile carbon level above 0.30% in the molten iron is not considered and thus production of only lower purity hydrogen-rich gas is possible.
U.S. Pat. No. 5,435,814 (1995) to Miller and Malone (Ashland) describes the general concept of a two-zone molten iron system process operating at high pressures, up to 100 atmospheres, with solid and liquid feed introduction below the surface of the molten iron and production of a hydrogen-rich and carbon monoxide rich gas streams. The disadvantages of the process described in this patent include:
There is no method described for handling the feedstock sulfur.
The importance of controlling the carbon level in the molten iron is not considered and thus production of only lower purity hydrogen-rich gas may be possible. The process is restricted to a particular method of circulating molten iron.
In summary, all of the above patents operate at atmospheric pressure and do not control carbon at a minimum of 0.3% in molten iron. Furthermore, the Rasor patents do not inject feed below the surface of the molten iron and are restricted to solids feeds. Furthermore, all the patents ignore sulfur in the feed or use slag to remove it.
One-zone molten metal gasifiers are disclosed by:
U.S. Pat. No. 4,496,369 (1985) to Torneman and U,S. Pat No. 4,511,372 (1985) to Axelsson in which coal or other liquid hydrocarbons are injected advantageously below the surface of the molten iron along with oxygen and water vapor to form a mixed hydrogen and carbon monoxide gas. Iron oxides are also added to the molten iron to act as a coolant for the melt. The primary objective of the invention is to produce gasified hydrocarbons and it is disclosed that production can be increased by operating at high pressures. The high pressures are not only economic because of the reduced size of equipment but it is also disclosed that high pressure decreases the degree of refractory wear in the reactor and the amount of dust carry-over in the gas from the reactor. In addition, it is disclosed that a higher sulfur level in the molten bath will also reduce the amount of dust carry-over from the reactor. The process disclosed cites the advantage of maintaining the carbon content of the bath below 0.8% carbon to reduce the amount of dust carry-over from the reactor.
The primary disadvantage of the process described in this patent include the lack of separate molten iron zones for gasification and thereby does not permit production of individual hydrogen-rich and carbon monoxide-rich gas streams.
U.S. Pat. Nos. 4,574,714 and 4,602,574 (1986) to Bach and Nagel in which solid or liquid toxic and/or lower value hydrocarbons are injected advantageously below the surface of the molten iron alloy, along with oxygen specifically to destroy the toxic compounds. With appropriate feeds a mixed hydrogen and carbon monoxide gas can be formed and C1 chemistry may be utilized to advantage at times to produce useful products. It is further disclosed that maintaining a carbon level of 0.5-6% carbon, preferably 2-3% carbon in tile molten metal is desired to prevent refractory degradation and facilitate reaction kinetics by providing a high concentration gradient for toxics destruction. Sulfur, when present in the feed, is removed via absorption in the slag. The disadvantages of the process described in this patent include.
The feedstocks are introduced to the molten iron single zone system for destruction as hazardous materials and not to produce hydrogen-rich or carbon monoxide-rich gases and thereby misses the advantages of feeding non-hazardous feedstocks.
Sulfur removal from the solid feed via reaction with and removal of slag from the equipment is complicated and expensive.
Operating pressure is nominally atmospheric, which is less economical to operate than equipment producing gases at elevated pressures; two atmospheres and above. Also, rotating gasification vessels on trunnions for slag removal makes operating at higher pressures impractical.
The importance of controlling the carbon level in the molten iron at more than 0.3% is not considered and thus production of only lower purity hydrogen-rich gas is possible.
The primary disadvantage of the process described in this patent include the lack of separate molten iron zones for gasification and thereby does not permit production of individual hydrogen-rich and carbon monoxide-rich gas streams.
The following foreign patents also disclose processes related to that of this application.
U.K. Patent 1,187,782 (1970) to Nixon discloses a reactor in which a hydrocarbon is introduced to one zone resulting in the production of a hydrogen-rich gas and oxygen is introduced into a second zone where the carbon which was dissolved in the first zone is burned with oxygen to give the exothermic heat to maintain the appropriate temperature in the first zone. It is noted that the two zone system as described has an advantage over hydrogen production in a single zone system which is operated in xe2x80x9cblocked outxe2x80x9d operations equivalent to that of a two zone system. It is further disclosed that sulfur present in the iron may be removed, purifying to some extent the iron. The disadvantages of the process described in this patent include:
Since no attempt is made to produce a carbon monoxide-rich gas in the oxidation zone, only a hydrogen-rich gas stream is produced.
Operating pressure is nominally atmospheric pressure, which is less economical to operate than equipment producing hydrogen at elevated pressures; two atmospheres and above.
The importance of controlling the carbon level above 0.3% in the molten iron is not considered and thus production of only lower purity hydrogen-rich gas is possible.
One embodiment of U.K. Patent 1,437,750 (1976) to Agarwal and Ahner describes producing a combustible gas containing a ratio of hydrogen to carbon monoxide of between 2:5 to 10:1 using a two-zone molten iron reactor with a coal feed to the top of one zone. Although the gases are produced in separate zones after further conversion with, for example, the water gas shift reaction, they are combined so the product from the system is a single combustible gas. Carbon concentrations in the molten iron are between 1 and 3% in the first zone and between 3 and 5% in the second zone. The disadvantages of the process described in this patent include:
The feedstocks are limited to solid hydrocarbons such as coal and do not include lower value hydrocarbon liquids or gases.
Since the solid hydrocarbon feeds are introduced above the surface of the molten iron, cracking of the feeds occurs such that a very impure hydrogen gas stream is produced because of the presence of cracked hydrocarbon gases.
Since the product gas from the oxidation zone is combined with the gas from the first zone, no attempt is made to produce a carbon monoxide-rich gas.
No disclosure is made concerning sulfur removal from the solid feed via reaction in the molten metal zones.
Operating pressure is nominally atmospheric pressure, which is less economical to operate that equipment producing hydrogen at elevated pressures, two atmospheres and above.
U.K. Patent 2,189,504 (1987) to Herforth describes a two-zone molten iron reactor in which low grade solids fuels are gasified in one zone and high grade solid fuels are gasified in the second zone. This permits the low grade solid fuels and waste materials to be consumed and produce a low quality off-gas where as the gasification of high grade fuels in the second zone permits production of a high quality off-gas unmixed with the low quality off-gas while still permitting destruction of the low grade fuels or waste materials. The sulfur is removed in the slag formed in the reactors. The disadvantages of the process described in this patent include:
The feedstocks are limited to solid hydrocarbons such as coal and do not include lower value hydrocarbon liquids or gases.
Since the solid hydrocarbon feeds are introduced above the surface of the molten iron, cracking of the feeds occurs such that a very impure hydrogen gas stream is produced because of the presence of cracked hydrocarbon gases.
No attempts are made to produce either a hydrogen-rich or carbon monoxide-rich off-gas.
Sulfur removal from the solid feed via reaction with and removal of slag from the equipment is complicated and expensive.
Operating pressure is nominally atmospheric pressure, which is less economical to operate that equipment producing hydrogen at elevated pressures; two atmospheres and above.
French Patent 2,186,524 (1974) to Vayssiere describes a two-zone molten iron system with a hydrogen-rich gas generated from hydrocarbons injected beneath the surface of the molten iron in one zone and either a carbon monoxide-rich gas or mixture of hydrogen and carbon monoxide gas generated by injecting oxygen or oxygen and hydrocarbon into the second zone. The disadvantages of the process described in this patent include:
There is no provision for removal of the sulfur in the feed.
Operating pressure is atmospheric pressure, which is less economical to operate that equipment producing gases at elevated pressures; two atmospheres and above.
The importance of controlling the carbon level above 0.3% in the molten iron is not considered.
In summary, while such systems referenced above may provide reasonable results, none of them effect the production of a separate hydrogen-rich stream and a separate carbon monoxide-rich stream at elevated pressures by feeding hydrocarbons below the surface of the molten iron and with controlled carbon contents of the molten metal above 0.3%. Furthermore, these systems either have no provision for handling feed sulfur or use a complicated and costly slag technique for sulfur removal. Sulfur capture in the slag requires slagging materials to be added to the molten metal zones and a more complicated means of regularly drawing off the sulfur containing slag. When sulfur is captured in slag the slag must then be disposed of, typically in uneconomic and environmentally unsound landfills.
Thus, our survey of prior practices indicates that the prior art has not combined the use of two zone molten iron gasifiers for separate hydrogen-rich and carbon monoxide-rich gas production, feed introduction below the molten iron surface, high pressure operation and carbon content control of the molten iron in the manner we have.
Broadly, this invention involves a process for producing in separate streams a hydrogen-rich gas and a carbon monoxide-rich gas from two molten metal zones and necessary ancillary equipment. Molten metal components are intended to include any molten material layer within a particular zone; e.g., molten metals, such as iron and its alloys, which are always present and slag components, if present, that would form a second molten layer with such molten metals. The molten metal employed in this invention is preferably molten iron but may be copper, zinc, especially chromium, manganese, or nickel, or other meltable metal in which carbon is somewhat soluble and which is at least 50% molten iron by weight.
In the first molten metal zone, a hydrocarbon feed in the form of a relatively dry gas or liquid or solid or solid-liquid slurry or atomized solid or liquid is fed beneath the molten metal surface and a hydrogen-rich gas is produced. By relatively dry is meant below 1% by weight of water. By introducing the feed below the surface of the molten metal substantially complete chemical reactions and conversions to hydrogen and carbon of the feed can be achieved. The carbon in the hydrocarbon feed dissolves in the molten metal.
In the second molten metal zone, into which molten metal from the first molten metal zone flows, all oxygen bearing stream is introduced to convert the carbon dissolved in the molten metal from the first zone into a carbon monoxide-rich gas stream which exits from above the molten metal bath in a gas stream separate from the hydrogen-rich gas stream from the first molten metal zone. Molten metal from which the carbon has been gasified by oxygen in the second zone is returned to the first molten metal zone.
Both molten metal zones are operated at elevated pressures, above two atmospheres, to reduce the size of the equipment need to produce and further treat, if necessary, the hydrogen-rich and carbon-monoxide rich gases. In addition, as disclosed is U.S. Pat. No. 4,511,372 incorporated by reference, operation at high pressures reduces dust carry over and wear on refractory walls of the vessel. Furthermore, the capital and operating costs of compression, of these gases to pressures at which they are utilized commercially are eliminated or substantially reduced.
Furthermore, in this process the amount of carbon in the molten iron to which the hydrocarbon feed is introduced is carefully controlled to above 0.3% to minimize formation of high levels of FeO, ferrous oxide, which could include a separate FeO phase. High levels of FeO will react with the carbon in the hydrocarbon feed and produce high levels of carbon monoxide, thereby contaminating the hydrogen-rich stream. If a separate phase of FeO is present it will attack the refractory of the vessels holding the molten iron. The amount of carbon in the molten iron should not normally exceed an tipper limit as determined by its solubility in molten iron.
This invention also includes having the hydrogen-rich and carbon monoxide-rich gases flowing from die molten metal zones through separate product gas lines to pass through successive downstream coolers, scrubbers or other gaseous impurity removal devices, and knock-out drums to cool the gases and to remove any solids and any condensed liquids from the gas streams. The gases may further be fed to proven scrubbers which remove hydrogen sulfide and other volatile sulfur compounds produced in the molten metal zones and emit a substantially sulfur-free and carbon oxide-free hydrogen-rich product gas and a substantially sulfur-free carbon monoxide-rich product gas.
Suitable feeds for the process include carbonaceous reactant feedstocks selected from the group consisting of: light gaseous hydrocarbons such as methane, ethane, propane, butane, natural gas, and refinery gas; heavier liquid hydrocarbons such as naphtha, kerosene, asphalt, hydrocarbon residua produced by distillation or other treatment of crude oil, fuel oil, cycle oil, slurry oil, gas oil, heavy crude oil, pitch, coal tars, coal distillates, natural tar, crude bottoms, and used crankcase oil; solid hydrocarbon such as coal, rubber, tar sand, oil shale, and hydrocarbon polymers; and mixtures of the foregoing.